Oxytocin the so-called "love hormone" is being increasingly shown to trigger a wide variety of physical and psychological effects in both women and men.
The hormone's influence on our behavior and physiology originates in the brain, where it's produced by the by a structure called the hypothalamus, and then transfers to the pituitary gland which releases into the bloodstream.. Like antennas picking up a signal, oxytocin receptors are found on cells throughout the body. Levels of the hormone tend to be higher during both stressful and socially bonding experiences, according to the American Psychological Association.
"It's like a hormone of attachment, you might say," said Carol Rinkleib Ellison, a clinical psychologist in private practice in Loomis, California and former assistant clinical psychiatry professor at the University of California, San Francisco. "It creates feelings of calm and closeness."
Thought scientists have long known about oxytocin's rolein breastfeeding and childbirth, "We're just learning more about it now," Ellison said.
A stream of studies in the last decade have focused on oxytocin's effects on body and mind. Here's a look at what we've learned.
Pregnant women with higher levels of oxytocin during their first trimester bonded more strongly with their babies after they were born, according to a 2007 study in the journal Psychological Science. And compared with other women, women with higher levels throughout their pregnancy and in the first month after birth reported engaging in more behaviors such as singing, feeding and bathing their infants in specific ways that promoted an exclusive relationship between the two, the study found.
Comparing urine levels of oxytocin and a related hormone called vasopressin in biological and adoptive children who lived in Russian and Romanian orphanages, researchers found that oxytocin rose in biological children after having contact with their mothers. The study, published in 2005 in the journal Proceedings of the National Academy of Sciences, showed that oxytocin levels remained static in the adoptive children in the same situation, suggesting a physiological basis for why some adoptive children have difficulty forming secure relationships.
Research done on prairie voles showed that those separated from their siblings exhibited signs of anxiety, stress and depression that abated after they were injected with oxytocin. The study, presented at a 2007 meeting of the Society for Neuroscience, indicated the hormone's effects were more evident under stressful situations.
A November study in the journal Proceedings of the National Academy of Sciences supported researchers' theory that oxytocin would amplify men's early memories of their mothers. In a group of 31 men, those who inhaled a synthetic version of the hormone found the hormone intensified fond memories of their mothers if their relationships had been positive. Those whose ties with their mom's had frayed downgraded their opinions after inhaling oxytocin, the study showed.
In its best understood role, oxytocin is released in large amounts during labor, intensifying the uterine contractions that open the cervix and allow the baby to pass through the birth canal. Physicians have been using synthetic oxytocin, also known by its brand name Pitocin, to induce or augment labor since the early 1900s. After birth, the hormone continues to stimulate uterine contractions that discourage hemorrhaging, and more is released when the nipples are stimulated during suckling, promoting the letdown of milk into the nipples.
Spontaneous erections in rats were observed after oxytocin was injected into their cerebrospinal fluid in a 2001 study in the journal Physiological Review. And a cocktail of brain chemicals that includes oxytocin is released in men during ejaculation. These chemicals can intensify bonding between sexual partners , though, Ellison noted, "it isn't the same for everyone."
"I think there is a variability," said Ellison, who also teaches sexuality classes to health professionals. "For people who can really get into the sensualness of hugging and cuddling, that is the hormone released in this process. For people who don't get into it, maybe they're not releasing the oxytocin. It may be a circular thing."
According to a 1999 article in the journal Progress in Brain Research, some studies indicate that oxytocin inhibits tolerance to addictive drugs, including opiates, cocaine and alcohol , and reduces withdrawal symptoms. "It's an antidote to craving," Ellison explained. "That craving (for drugs), that hunger, is probably eased with this hormone. It's involved with the satisfaction of hunger."
A February study in the journal Proceedings of the National Academy of Sciences showed that inhaling oxytocin significantly improved the ability of people with autism to interact with others. Previous studies indicated natural oxytocin levels were lower in those with autism, a developmental disorder characterized by difficulties in communication and social relationships. Oxytocin also reduced autistic individuals' fear of others, researchers said.
A June study in the journal Science suggested oxytocin triggers defensive aggression against outsiders who might threaten someone's social group, such as in soldiers who defend their comrades. Prior animal studies had shown that the hormone promotes protectionist behavior, but this research was the first to demonstrate a similar effect in humans.
Oxytocin released in the brain under stress-free conditions naturally promotes sleep , according to a 2003 study in the journal Regulatory Peptides. Ellison said this link makes sense because oxytocin counters the effects of cortisol, which is the known as the stress hormone. "It has a calming effect," she said. "It leaves you feeling tranquil and loving, and certainly that helps our path to sleep."
In a 2007 study in the journal Public Library of Science ONE, participants inhaled oxytocin or a placebo through their noses, and then were given a decision on how to split money with a stranger. Those on oxytocin were 80 percent more generous, researchers said, and the hormone seemed to affect their sense of altruism as well.